The present invention relates to a light-emitting device like a semiconductor laser device, and more particularly relates to a semiconductor light-emitting device for emitting radiation in the ultraviolet to blue regions. The present invention also relates to a method for fabricating the semi-conductor light-emitting device and to an optical disk apparatus using the light-emitting device.
In recent years, semiconductor light-emitting devices that can emit radiation at short wavelengths ranging from the ultraviolet to blue regions, or semiconductor laser devices, in particular, have been researched and developed vigorously. This is because such light-emitting devices are expected to further increase the recording density of optical disks or the resolution of laser printers and are applicable to optical measuring instruments, medical equipment, display devices, illuminators and so on.
Examples of semiconductor materials that can emit radiation at such short wavelengths include Group III nitride semiconductors. For instance, a semiconductor laser device with a multiple quantum well active layer, which is a stack of silicon (Si)-doped GaInN/GaInN layers, can oscillate continuously at a wavelength of about 401 nm and at room temperature and can operate for as long as about 3,000 hours under the conditions that the ambient temperature is 20° C. and the output power thereof is 2 mW. See Japanese Journal of Applied Physics, Vol. 36 (1997), pp. 1568-1571, for example.
Group III nitride semiconductor crystals are generally grown by a metalorganic vapor phase epitaxy (MOVPE) process. For example, Japanese Laid-Open Publication No. 6-196757 discloses a method of growing a semiconductor layer of GaInN of excellent crystal quality on a semiconductor layer of GaN by using nitrogen as a carrier gas.
The known method of producing a Group III nitride semiconductor, however, is disadvantageous in that pits are created in the GaInN/GaN multiple quantum well structure thereof (to be an active layer) at as high a density as 108 to 109 cm−2 as described in Applied Physics Letters, Vol. 72 (1998), pp. 710-712, for example.
Those pits adversely affect the operation characteristics of a light-emitting device, e.g., raises the threshold value, at which the laser device starts to oscillate, or lowers the reliability thereof. This is because the existence of the pits not only decreases the luminous efficacy, but also causes localized levels by making the composition of In non-uniform, constitutes a source of diffusion of In being grown or results in scattering or absorption loss in an optical waveguide.
To obtain a Group III nitride semiconductor light-emitting device, or semiconductor laser device, in particular, with characteristics practically applicable to an optical disk apparatus, for example, the composition of In within the GaInN well layer thereof should be uniformized. In addition, each multiple quantum well layer should be of uniform quality and be sufficiently planarized.
Moreover, the structure of the device should be modified such that electrons, which are injected from an n-type conductive layer into the quantum well layer, can be injected into the active layer efficiently and uniformly without overflowing into a p-type conductive layer during the operation of the device.